Improved thermal stability and hole mobilities in a strained-Si/strained-Si1−yGey/strained-Si heterostructure grown on a relaxed Si1−xGex buffer

A dual channel heterostructure consisting of strained-Si/strained-Si1−yGey on relaxed Si1−xGex (y > x), provides a platform for fabricating metal-oxide-semiconductor field-effect transistors (MOSFETs) with high hole mobilities (μeff) which depend directly on Ge concentration and strain in the strained-Si1−yGey layer. Ge out-diffuses from the strained-Si1−yGey layer into relaxed Si1−xGex during high temperature processing, reducing peak Ge concentration and strain in the strained-Si1−yGey layer and degrades hole μeff in these dual channel heterostructures. A heterostructure consisting of strained-Si/strained-Si1−yGey/strained-Si, referred to as a trilayer heterostructure, grown on relaxed Si1−xGex has much reduced Ge out-flux from the strained-Si1−yGey layer and retains higher μeff after thermal processing. Improved hole μeff over similar dual channel heterostructures is also observed in this heterostructure. This could be a result of preventing the hole wavefunction tunneling into the low μeff relaxed Si1−xGex layer due to the additional valence band offset provided by the underlying strained-Si layer. A diffusion coefficient has been formulated and implemented in a finite difference scheme for predicting the thermal budget of the strained SiGe heterostructures. It shows that the trilayer heterostructures have superior thermal budgets at higher Ge concentrations. Ring-shaped MOSFETs were fabricated on both platforms and subjected to various processing temperatures in order to compare the extent of μeff reduction with thermal budget. Hole μeff enhancements are retained to a much higher extent in a trilayer heterostructure after high temperature processing as compared to a dual channel heterostructure. The improved thermal stability and hole μeff of a trilayer heterostructure makes it an ideal platform for fabricating high μeff MOSFETs that can be processed over higher temperatures without significant losses in hole μeff.